The Provinces of Prussia – Flags in Lexicon and Shop https://www.flaggenlexikon.de/fdtlhipp_me.htm
The State of Prussia was divided into 14 provinces until 1919 and into 13 provinces until 1945. All of them had their own flag. These flags – which can also be ordered on these pages – showed the respective historical national colours.
The Provinces of Prussia in alphabetical Order (from ca. 1850) • Town of Berlin • Brandenburg